1. Field of the Invention
This invention relates to an active-matrix liquid crystal color display panel having a triangular pixel arrangement, and more particularly to an improvement for high quality color image.
2. Description of the Related Art
Recently, a liquid crystal color display panel has been remarkably progressed and is demanded to image a high quality color picture. An active-matrix liquid crystal display panel using thin film field effect transistors (hereinafter, referred to TFT) as switches has been recognized, in recent years, as preferable to image a fine high quality color image.
One pixel element is formed of a TFT and a display electrode. A plurality of pixel elements are disposed in a matrix form on a transparent glass substrate. The TFT's are driven by access signal buses (or scanning buses) and data signal buses (or video signal buses) disposed between pixel elements to form a lattice. Scanning signals are applied to TFT's through the access buses. Video signals are applied to the data buses to be supplied to selected display electrodes through TFT's which are turned on by the scanning signals.
In a color display, one color pixel element has three or four pixel elements (hereinafter, referred to cell element for imaging primary color components. Therefore, compared to a black-and-white display, number of cell elements required is more than three times. The larger the number of color pixel elements (that is, the number of cell elements), the higher the resolution of the obtained color image becomes. However, if the number of cell elements is increased, the time duration for driving one cell element becomes short to decrease the effective voltage applicable to liquid crystal, resulting in a poor quality of imaged picture. In this point of view, the activematrix liquid crystal color display panel is superior to other types of liquid crystal display panel. The activematrix type is small in decrease of the effective voltage applied to liquid crystal, if the driving time duration becomes short. However, in a case where the number of cell elements are greatly increased for obtaining a high quality color image, the driving time duration becomes too short to apply the sufficient effective voltage to the liquid crystal in the active-matrix type color display panel.
An arrangement for prolonging this driving time duration is to form one cell element with one display electrode and two TFT's and to successively drive those two TFT's by adjacent two access buses. By this arrangement, the time duration for applying a video signal to one display electrode becomes double, compared to the case where one TFT is coupled to one display electrode. However, since one access bus simultaneously drives adjacent two cell elements in the direction parallel with data signal bus, all the cell elements in the direction parallel with the data bus have to receive a video signal of the same color. This means the primary color pixel arrangement is limited to be a stripe type. The stripe type color pixel arrangement has the same color cell elements in a line and is poor in image quality.
The image quality is improved by the triangular color pixel arrangement. K. Noguchi et al. proposed one improvement for the triangular color pixel element in U.S. patent application (Ser. No. 823,104) filed on Jan. 27, 1986. One cell element in one color is divided into two parts each having one TFT and one display electrode. The drain electrodes and gate electrodes of the two TFT's are connected to the same data bus and the same access bus. The two-part-set cell elements are connected to every access buses. But every other two-part-set cell elements are disposed on one side of the access buses, while the other two-part-set cell element are disposed on the other side of the access buses. Two two-part-set cell elements on one side of the access bus and one two-part-set cell element on the other side of the access bus form one color pixel element having a triangular shape. The arrangement of the color pixel elements along the access buses is shifted with a half pitch of one color pixel element between adjacent two access buses. This arrangement images a high quality picture. However, there is one drawback of a line-defect which appears if one access bus or data bus has an opencircuit introduced in a manufacturing process. This linedefect spoils the imaged picture.